Concentration and Velocity Measurements in Suspensions Flowing through a Rectangular Channel

A method to measure particle concentrations in dilute to moderately concentrated transparent liquid-solid suspensions was developed. The method uses a dual-beam laser-Doppler anemometer system. Particle concentrations were determined by counting the number of signals from fluorescently-dyed tracer particles per unit time. Using this method, both the velocities and concentrations of suspended particles were obtained. Velocity and concentration measurements were made in suspensions of neutrally-buoyant polystyrene-divinylbenzene spheres 27, 50, and 70 µm in diameter flowing in a rectangular channel with 800 µm spacing between walls. Measurements were made in the central plane of the flow channel, thus approximating two-dimensional flow. Dilute-suspension (particle volume fraction Φm = 0.001) experiments were performed both at Reynolds numbers that were low enough so that inertial effects were unimportant (particle Reynolds number Rep < 10-3), and Reynolds numbers at which inertial effects were significant (Rep = 10-3-10-1). Concentrated-suspension (Φm = 0.02-0.25) experiments were done at low Rep only. For the dilute suspensions at low Re, small peaks in the concentration distributions were found near the flow channel walls, which were attributed to an entrance effect. At higher Re, lateral migration of particles due to inertia was observed. The spheres migrated toward a lateral equilibrium position about 0.6 times the distance from the flow channel centerline to the walls, as demonstrated previously by others. The measured concentration profiles were compared to a previously published theory for the lateral migration of a single sphere, and it was found that the peaks near the equilibrium positions in the measured concentration distributions were somewhat broader than those predicted by the theory, presumably due to particle-particle interactions. Non-uniform concentration profiles were observed for the concentrated suspensions. The concentration profiles became somewhat peaked in the center of the channel as Φm and/or sphere diameter increased, particularly for the 50 and 70 µm sphere suspensions, and for these suspensions the velocity profiles were blunted for Φm ≥ 0.10. A model velocity profile calculation indicated that the shape of the concentration profiles could satisfactorily account for the shape of the velocity profiles, except possibly for the suspensions of 70 µm spheres.